Transistors are commonly used in integrated circuits. FIG. 1a shows a conventional n-type metal oxide semiconductor (MOS) transistor 110 formed on an active region of a substrate 105. A p-well 108 is located in active region of the substrate. Shallow trench isolations 180 are used to isolate the active region from other device regions. The transistor has source/drain diffusion regions 125a-b adjacent to a gate 130, which includes a gate electrode 131 over a gate dielectric layer 132. Located on the gate sidewalls are dielectric sidewall spacers 160. Metal silicide contacts 140 and 141 are provided on the surface of the substrate in the source/drain regions and gate electrode.
Nickel silicide or nickel alloy silicides have been extensively used in contacts due to low sheet resistance properties. Nickel (Ni) atoms, however, are highly diffusive. During processing, Ni atoms can diffuse beneath the spacers, resulting in the formation of Ni pipes 148. Ni pipes can lead to junction leakage, negatively affecting device performance or functionality. FIG. 1b shows a TEM image of nickel pipes 148. As shown, such nickel pipes can result in gate-to-source or gate-to-drain short circuits.
In view of the foregoing, it is desirable to provide transistors with metal silicide contacts which reduce or minimize the formation of metal pipes.